1. Field of the Invention
The present invention relates to an image forming apparatus which: forms an electrostatic image on an image-bearing member in accordance with image data of a manuscript by, for example, an electrophotographic printing method or an electrostatic recording method; develops the electrostatic image with a developer containing colored toner charged to predetermined polarity to provide a toner image; transfers the toner image on a recording material; and fixes the image to provide a color image having gloss. The present invention relates to an image forming apparatus such as a printer, a copying machine, or a facsimile.
2. Related Background Art
FIG. 1 shows an example of a color image forming apparatus according to an electrophotographic printing method. The image forming apparatus of this example will be briefly described on the basis of FIG. 1.
A color electrophotographic image forming apparatus 100 of this example has five image forming portions P (Pa, Pb, Pc, Pd, and Pe) arranged in parallel with one another in a horizontal direction. Each of the image forming portions P (Pa, Pb, Pc, Pd, and Pe) has the corresponding one of drum-shaped electrophotographic photosensitive members (hereinafter referred to as “photosensitive drums”) 1 (1a, 1b, 1c, 1d, and 1e) as an image-bearing member.
Each of the photosensitive drums 1 (1a, 1b, 1c, 1d, and 1e) is rotated clockwise by driving means in FIG. 1. In addition, each of the photosensitive drums 1 (1a, 1b, 1c, 1d, and 1e) has the corresponding one of charging devices 2 (2a, 2b, 2c, 2d, and 2e) for uniformly charging the surfaces of the photosensitive drums 1, the corresponding one of exposing devices 3 (3a, 3b, 3c, 3d, and 3e), the corresponding one of developing devices 4 (4a, 4b, 4c, 4d, and 4e), and the corresponding one of cleaning devices 5 (5a, 5b, 5c, 5d, and 5e) arranged on its periphery.
In addition, a conveying belt 7 serving as recording material conveying means for conveying a recording material S to each of the image forming portions P (Pa, Pb, Pc, Pd, and Pe) is arranged. The conveying belt 7 is tensioned between a driver roller 81 and support rollers 82 and 83, and is rotated in the direction indicated by an arrow.
In each of the image forming portions P (Pa, Pb, Pc, Pd, and Pe), the surface of any one of the photosensitive drums 1 (1a, 1b, 1c, 1d, and 1e) uniformly charged by the corresponding one of the charging devices 2 (2a, 2b, 2c, 2d, and 2e) is irradiated with a light image by the corresponding one of the exposing devices 3 (3a, 3b, 3c, 3d, and 3e), whereby an electrostatic latent image is formed.
The electrostatic latent image of each of the photosensitive drums 1 (1a, 1b, 1c, 1d, and 1e) is developed by the corresponding one of the developing devices 4 (4a, 4b, 4c, 4d, and 4e) to be a visible image, that is, a toner image.
That is, each of the developing devices 4 (4a, 4b, 4c, 4d, and 4e) is filled with a predetermined amount of cyan, magenta, yellow, black, or transparent toner as a developer by a supplying device (not shown). The developing devices 4 (4a, 4b, 4c, 4d, and 4e) develop the latent images on the photosensitive drums 1 (1a, 1b, 1c, 1d, and 1e) to visualize the images as a cyan toner image, a magenta toner image, a yellow toner image, a black toner image, and a transparent toner image.
The recording material S is stored in a recording material cassette 10, and is supplied from the cassette to the conveying belt 7 via multiple conveying rollers 11 and a registration roller 12. The recording material S is conveyed by the conveying belt 7 to be sequentially fed to transfer portions opposite to the photosensitive drums 1 (1a, 1b, 1c, 1d, and 1e). The toner images are transferred on the material by transfer blades 6 (6a, 6b, 6c, 6d, and 6e) as transfer means provided for the transfer portions.
Next, the recording material S on which the toner images have been transferred is separated from the conveying belt 7, and the separated recording material S is conveyed by a conveying portion 13 to a fixing device 9.
The recording material S on which the four color toner images and the transparent toner image have been transferred is subjected to fixation so that the mixing of the toner images and the fixation of the images to the recording material S are performed. Thus, a full-color copy image is formed, and is discharged to a sheet discharge tray 14.
FIG. 2 shows an example of each of the developing devices 4 (4a, 4b, 4c, 4d, and 4e) used in the image forming apparatus having the above construction. All the developing devices 4a, 4b, 4c, 4d, and 4e have the same construction. The developing devices 4 will be described.
In this example, each of the developing devices 4 is provided with a developing sleeve 40 as a developer carrying member, a magnet roller 41, a regulating member 42, developer conveying screws 43 and 44, and the like.
The developing sleeve 40 has the magnet roller 41 provided with multiple magnetic poles (S1, N1, S2, N2, and N3) fixed and included therein, and is rotated while a predetermined developing interval is maintained between the sleeve and the peripheral surface of the one of the photosensitive drums 1. The regulating member 42 has rigidity and magnetism, and may be arranged in various ways. For example, the member may be arranged while a predetermined interval is maintained between the member and the developing sleeve 40, or may be brought into pressure contact with the developing sleeve 40 under a predetermined load with no developer interposed between the member and the sleeve.
Any one of development methods can be used. To be specific, a preferable method involves: applying an alternating voltage to the developing sleeve 40 to form an alternating electric field in a developing region A where the developing sleeve 40 and the photosensitive drums 1 are opposite to each other; and performing development in a state where a magnetic brush is in contact with the photosensitive drum 1.
A distance between the developing sleeve 40 and the photosensitive drum 1 (an S-D distance) is desirably 100 to 1,000 μm for preventing carrier adhesion and for improving dot reproducibility. When the distance is smaller than 100 μm, a developer is apt to be insufficiently supplied, so an image density reduces. When the distance exceeds 1,000 μm, a line of magnetic force from the developing magnetic pole S1 expands. As a result, the density of magnetic brushes reduces, dot reproducibility deteriorates, or a binding force on a carrier weakens, so carrier adhesion is apt to occur.
The peak-to-peak voltage of the alternating electric field is preferably 300 to 3,000 V, and the frequency of the alternating electric field is 500 to 10,000 Hz. Each of the peak-to-peak voltage and the frequency can be appropriately selected and used in accordance with a process. In this case, examples of the waveform of an alternating bias for forming the alternating electric field include a triangular wave, a rectangular wave, a sinusoidal wave, and a waveform obtained by changing a duty ratio. Development is preferably performed by applying a developing bias voltage (an intermittent alternating superimposed voltage) having a discontinuous alternating bias voltage to the developing sleeve in order to cope with a change in rate at which a toner image is formed. When the applied voltage is lower than 300 V, a sufficient image density is hardly obtained, and it may be impossible to collect the fog toner of a non-image portion favorably. When the voltage exceeds 3,000 V, a latent image is disturbed through a magnetic brush, so image quality may reduce.
In addition, the use of a dual-component developer having favorably charged toner can reduce a fog removal voltage (Vback), so the primary charging of each of the photosensitive drums 1 can be reduced. As a result, the lifetime of each of the photosensitive drums 1 can be lengthened. A value for Vback is preferably 200 V or less, or more preferably 150 V or less although the preferable range varies depending on a developing system. A development contrast of 100 to 400 V is used so that a sufficient image density can be obtained. The development contrast is preferably as high as possible, and is preferably 300 V or more in order that the gradation of the halftone of an image may be stabilized.
When the frequency is lower than 500 Hz, sufficient vibration is not applied upon return of the toner in contact with each of the photosensitive drums 1 to the developing sleeve, so fog is apt to occur although the degree of fog is related to a process speed. When the frequency exceeds 10,000 Hz, toner cannot follow an electric field, so image quality is apt to reduce.
In such conventional image forming apparatus as described above, no toner is used in a white portion in a produced recorded image. The optical properties of paper as a recording material directly determine the visual characteristics of the recorded image such as a color and gloss. On the other hand, most of the visual characteristics of a portion where large amounts of black toner or C, M, Y, and K-color toners (such as a concentrated brown toner) are superimposed and recorded are determined by the optical properties of the toners.
With regard to the gloss of a recorded image outputted by such image forming apparatus as described above, for example, in the above-described case of white and black, black generally has higher gloss than that of white. This is because the gloss of toner is generally higher than that of paper as described above.
As a result, there arises a problem in that image quality is significantly impaired by a difference in gloss between pixels in an output image.
Furthermore, the height of toner at a portion having a high density is about 5 to 10 μm, so there arises a problem in that the irregularities of a toner image are so remarkable that image quality is reduced.
As an image forming apparatus taking such gloss and irregularities of toner as described above in consideration, as described in, for example, Japanese Patent Application Laid-Open No. H07-266614, there has been proposed a method involving: calculating the height of toner at an image portion to be formed on the surface of a recording material from image data by a toner height calculating portion; calculating the amount of transparent toner having gloss to be printed on each portion from a difference between the height of the toner at the image portion and the maximum value for the toner height by a transparent toner print amount calculating portion; printing the transparent toner in an amount needed for eliminating the irregularities of the toner on a toner image to provide a recorded article; imparting gloss to the recorded article; and eliminating surface irregularities to provide a glossy image.
However, the method involving smoothing a surface by transparent toner involves the formation of an image by the same amount of the transparent toner as that of the maximum toner amount of colored toner.
In other words, the same amount of the transparent toner as that of the maximum toner amount produced by appropriately superimposing four colors (cyan, magenta, yellow, and black) must be used to form an image at a portion where the image is formed by the transparent toner.
For example, in the case where image processing involving the superimposition of two or more colored toners is used, when the maximum toner amount of each colored toner is 0.5 mg/cm2, the transparent toner must be used in an amount of 1.0 mg/cm2 or more once for image formation.
An image forming method similar to the image formation by colored toner involves difficulty in forming an image at one image forming portion by a larger amount of transparent toner as that of colored toner.
Detailed description will be given below with reference to FIG. 3.
Description will be given by taking, as an example, the electric potential of each of the photosensitive drums 1 and the electric potential of the developing sleeve 40 in ordinary image formation. When the charge amount of colored toner per unit weight (hereinafter referred to as the triboelectricity) in an environment (23° C., 50% Rh) is about −30 μC/g, a high voltage is applied to each of the charging devices 2 so that the surface electric potential of the photosensitive drum 1 is controlled to −650 V.
Meanwhile, an alternating bias obtained by superimposing a DC component of −500 V and an AC component of 1.2 kV p-p on each other is applied to the developing sleeve 40. When each of the photosensitive drums 1 is exposed to laser, the drum shows a light electric potential of −100 V at a site where an electrostatic latent image to serve as an image having the maximum density is formed.
Therefore, a development contrast is set to about 400 V when colored toner having a maximum bearing amount of 0.5 mg/cm2 is developed.
In the case where transparent toner is used in such construction, if it is assumed that the triboelectricity of the transparent toner is about −30 μC/g as in the case of the triboelectricity of colored toner, an electric potential difference needed for developing 1.0 mg/cm2 of the transparent toner is about 800 V. In this case, a latent image electric potential of about 900 V must be formed on each of the photosensitive drums 1. Providing the ordinary photosensitive drum 1 with such high electric potential is not realistic because charging performance is not stable owing to the performance of the photosensitive drum.
For example, the following control is also available: a development contrast in accordance with the maximum electric potential of the charge amount of each of the photosensitive drums 1 is 400 V, the value is defined as a value set for transparent toner, and the development contrast of colored toner is set to 200 V in accordance with the maximum toner amount of the colored toner. In this case, however, there arises a problem in that the gradation of the halftone of the colored toner is unstable.